This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
In hydrocarbon exploration, development, and/or production operations, different types of subsurface models may be used to represent subsurface structures, which may include a description of subsurface structures and material properties for the subsurface region. For example, the subsurface model may comprise one or more of a geologic model, a geomechanical model, or a reservoir model. The subsurface model may represent measured or interpreted data for the subsurface region and may include objects (e.g., horizons, faults, surfaces, volumes, and the like). The subsurface model may also be discretized with a mesh or a grid that includes nodes and forms faces or cells (e.g., voxels or mesh elements) within the model. By way of example, the subsurface model may be created from a structural framework (e.g., organization of objects) and provide defined compartments or subvolumes. The geologic model may represent measured or interpreted data for the subsurface region, such as seismic data and well log data. The geologic model may be within a physical space or domain and may have material properties, such as rock properties. The reservoir model may be used to simulate the flow of fluids within the subsurface region. Accordingly, the reservoir model may use the same mesh and/or cells as other models or may resample or upscale the mesh and/or cells to lessen the computations for simulating fluid flow within the subsurface.
The development of accurate subsurface models can be problematic. For example, in geomechanical simulations, stresses and strains, such as extensional strain in the brittle crust of the earth, are typically accommodated by opening mode natural fractures, known as joints that grow perpendicular to the most tensile principal stress. Joints can form under a combination of macromechanically driven loading conditions, such as elevated pore pressures, folding of geological strata, vertical stacking of heterogeneous layers, slip along preexisting faults, and others. Although the existence of microflaws may play a role in fracture initiation and propagation, mechanically driven factors dominate the resulting natural fracture patterns. Natural fractures occur in varying orientations, occur in clusters, have different propagating lengths, and have various apertures. One of the main difficulties of fracture prediction is to be able to characterize such properties.
Because joints may affect the movement, storage, and recovery of hydrocarbons, a substantial effort has been focused on predicting the characteristics of subsurface systems. The most commonly sought characteristics include the intensity, aperture, and orientation of the dominant set of joints. In data rich fields, fracture characterization efforts include analyzing the image log and core data, and sometimes investigating seismic attributes such as the azimuthal variation of P-wave velocities or amplitude vs offset. However, in fields where subsurface data are limited, or in the early stages of exploration, fracture characteristics are usually predicted from 2-D or 3-D restoration or curvature analyses which have limited power for fracture prediction. See, e.g., Keating D. P., and Fischer, M. P., 2008, An experimental evaluation of the curvature-strain relation in fault-related folds, AAPG Bulletin v. 92, no. (7), 20, p. 869-884.
From a computational point of view, historically pure continuum models have been favored over discontinuum models for modeling naturally occurring fractures. This is due to their computational efficiency and ease of implementation. However, continuum models are unable to realize true material separation, ultimately exhibiting regions of zero strength and effectively eliminating meaningful post failure interaction, which may include predicting multiple sets of fractures. Indeed, all fractures in quasi-brittle materials are associated with extension and parting of material planes. In this sense, the principal problem with continuum models is that the resultant models are “too stiff” when no a priori knowledge of crack initiation or distribution is available. To develop computational methodologies with truly predictive capabilities, emphasis must be placed on the evolution of material response throughout the deformation process.
Currently such evolving material response is homogenized and is represented using phenomenological models. Given that discontinuities in geologic media (e.g., rocks) manifest with deformation and perturb the stress field (i.e. by introducing stress concentrations and additional kinematic freedom), accurate representation of their influence within computational models can be achieved only when the continuum models evolve naturally into a discontinuum model.
Although modeling fractures in rock masses may be of practical importance, this problem is among the most difficult to solve because the fracture state evolves continuously and is not known a priori. At any time during the deformation process, fractures may be open, partly closed, or completely closed, and, if open, may close under compression. Furthermore, parts of closed fractures may begin to slip and later stick and initiate additional discrete fracture sets such as splaying or wing fractures. Thus, any numerical approach that aims to predict fracture characteristics (such as length, aperture, and intensity) should be able to capture the sequential evolution of discontinuities within a model, preferably starting from a continuum state without a priori knowledge of the microflaw distribution. Therefore, what is needed is a novel methodology that predicts naturally occurring fractures and damage in subsurface regions.
Various approaches have been developed to create subsurface models. For example, U.S. Patent Application Publication No. 2011/077918 describes various techniques that have been used to predict natural fractures and associated damage in a subsurface region, which include: analytical methods, curvature analysis, restoration analysis, stochastic techniques, continuum mechanical analysis, and mechanical methods. In U.S. Patent Application Publication No. 2011/077918, a method of predicting naturally occurring fractures and damage as observed in a subsurface region is described. The methodology integrates a variety of fracture prediction tools and a hybrid finite-discrete element method of Finite Element Method (FEM) and Discrete Element Method (DEM) that simulates the transition of rock from a continuous to a discontinuous state as observed in nature. Unfortunately, this method is computationally inefficient because the fine mesh and coarse mesh are simulated together.
Accordingly, there remains a need in the industry for methods and systems that are more efficient and may lessen problems associated with forming a subsurface model for use in hydrocarbon operations. Further, a need remains for an enhanced method to simulate the presence, distribution, characteristics, and subsurface properties, such as flow properties of natural fractures in the subsurface, using an iterative global-local mechanical modeling approach that combines simulating a reservoir deformation history with explicit fracture prediction on bed scale. In addition, a need remains for an enhanced method for modeling flow-based effective flow properties and local calibration of rock heterogeneity, which may also provide characterizations of natural fractures in the subsurface away from locations of measurements. The present techniques provide a method and apparatus that overcome one or more of the deficiencies discussed above.